MORE THAN three decades after the first neonatal intensive care nurseries were founded, improvements in technology and care save tens of thousands of babies' lives each year. Yet premature birth still is the most common cause of death in newborn infants and a major cause of lifelong disability. For the smallest and earliest-born premature infants, doctors, nurses and parents still must ask our most difficult question: "Is this baby too fragile to survive?"

For the parents, who must choose whether or not to try to resuscitate a baby born too prematurely, the agony comes from wondering whether they dare hope for a good outcome. When that hope is slim, the parents' decision to withhold life support can mean that they still can protect the child from pain, yet have an opportunity for a loving farewell. Parents need advice from the baby's physician to make that choice.

This advice is one of the most difficult, and perhaps most important, tasks for a neonatologist. The physician can and should explain the facts, drawn from experience, and offer an opinion. Yet, when a baby is at the edge of viability, it can be very difficult to form that opinion, in part because the whole history of neonatal intensive care has been a matter of re-defining "hopeless" cases.

To understand the difficulty, the reader needs to know more about predicting the risk of a poor outcome. A baby is considered premature if it is born earlier than 37 weeks gestational age -- that is, 37 weeks after conception. However, most deaths and most serious disabilities occur in infants born before 32 weeks gestational age. Infants born before a certain gestational age are not able to survive outside the womb, no matter how sophisticated the technology.

Thirty-five years ago, a baby born at 30 weeks of gestation was in that marginal zone for survival. With the current state of technology,

babies born sooner than 23 weeks' completed gestational age

almost never survive outside the womb. Those born at 23 to 24

weeks of gestation are in a marginal zone where the risk of death

is high and the quality of life for survivors usually is poor. Extra

time in the womb above 24 weeks makes a difference in chances

of survival and chances of a favorable outcome.

Premature birth causes death and disability to some babies because major organ systems are not developed to the point where they can function outside the womb. Until the early 1970s, tens of thousands of babies died each year because their lungs were too immature to support breathing. (One of these infants was President and Jacqueline Kennedy's third child.) Medical and life-support technology has improved to support premature lungs and other organs, but the earlier the gestational age, the greater the risk of deadly damage. Organ damage, particularly as it affects the developing brain, can lead to permanent disabilities, including cerebral palsy, mental retardation, deafness and blindness. The results can be devastating for both the baby and the family.

Another indication of prematurity, in addition to gestational age, is low birth weight. Thirty-five years ago, half of all premature infants who weighed 1,000 to 1,250 grams (2 to 2 3/4 pounds) at birth died. Today, 95 percent survive. For babies weighing 1,500 grams and more, care in an experienced nursery means a very high chance that the child will live with no major neurological defects.

Overall, the technology that saves premature babies' lives is cost effective. However, an infant in the marginal zone who does survive will require months of intensive care. Some health-policy experts question this very large commitment of resources.

The ideal care for premature infants would be to prevent early delivery. How ever, despite years of research, there has been no reduction in the overall incidence of premature birth. Some progress is being made in delaying a premature baby's delivery. Delaying delivery for even one or two weeks can mean a substantial improvement in the outcome for the infant.

The availability of this technology still raises difficult ethical and health-policy questions. Should one deploy this invasive and expensive technology when the chances for success are vanishingly small? Should a physician recommend resuscitation for such a baby at birth? Should a physician recommend an emergency cesarean section at a marginal gestational age when there are signs that a fetus is in severe distress?

Clinicians trying to make the most sensible recommendations to parents who are torn between two questions: "Is it time to attempt to move the barrier of viability down by a week?" versus "Haven't we reached the irreducible limit?" Parents don't have to accept the doctor's recommendation, but the doctor owes them an opinion.